webadmin: Check for NPE in AsyncDataProvider:GetInterfaceOptionsForEditNetwork

There's a possible branch of statement, that if executed
a null value will be dereferenced. this patch verifies that the var
is not null before using it.

Change-Id: Iba10d8a40e414e18fa6a87024dad9e1ed6a69bc7
Signed-off-by: Asaf Shakarchi <asaf@redhat.com>